The proposed studies will take a basic mechanistic approach to understanding ablation of dental hard tissue during irradiation with infrared (IR) lasers. The overall objective of these studies is to determine the specific laser irradiation conditions, namely wavelength, irradiation fluence, and pulse duration, that maximize the efficiency of dental hard tissue ablation while minimizing the heat deposition in the tooth. The central hypothesis that will be tested in this proposal is that IR laser radiation tuned to specific IR absorption bands of dental hard tissue with pulse durations commensurate with the thermal relation time of the heat deposited at the tissue surface (50-500 microsec), can efficiently ablate dental hard tissue while minimizing peripheral thermal damage. This hypothesis will be tested by the following specific aims: #1. To test the hypothesis that the mechanism of IR laser dental hard tissue ablation at 9-11 micrometer wavelengths is predominately photothermal while in the 2.7 to 3.0 micrometer absorption region ablation is dominated by inertially-confined, water-mediated ablation for pulse durations of approximately 100 microseconds: (1) various methods of surface analysis will be used to determine the ejection anisotropy, particulate size distribution, porosity, stoichiometry, and crystallinity of ablated material. Similar methods will be used to analyze the nature of the irradiated tissue surface after ablation. (ii) Optical spectroscopy, time-resolved imaging, and mass spectroscopy will be used to study the evolution of the plume of ablated material, detect the onset of plasma formation, and probe particle-plume interactions. #2: To test the hypothesis that clinically relevant ablation rates and efficiencies can be realized at the highly absorbed laser wavelengths between 2.7 and 3.0 and 9-11 micrometer without thermal or acoustic insult to healthy tissue: (i) the ablation thresholds and efficiency of carious and noncarious enamel and dentin ablation will be determined using mass spectroscopy, photoacoustics, and tissue perforation measurements. (ii) Radiometer, reflectance, calorimetry, and microthermocouple temperature measurements in conjunction with numerical simulations will determine the heat redistribution and rate of heat accumulation in the tooth during multiple pulse laser irradiation with and without air/water cooling. (iii) Polarized light microscopy, X-ray tomography, scanning electron microscopy, and transient stress measurements will be used to examine tissue peripheral to ablation sites to assess the potential for thermal and stress induced damage. #3: To test the hypothesis that the laser conditions indicated in the above specific aims are clinically applicable for the removal of carious tissue and to prepare the cavity for restoration. This study will provide, after accomplishing the specific aims, the laser parameters, namely wavelength, fluence, repetition rate, and pulse duration, to be subsequently tested clinically for safety and efficacy. If those clinical studies are a success, it is likely that they will lead to the use of new conservative tissue removal techniques based on a novel IR laser system, developed as a result of the proposed fundamental studies, which may markedly reduce the amount of healthy tissue loss that is generally associated with cavity preparations.